Leonard C Suskin's musings on writing, parenthood, and the wonderful world of commercial AV.

Monday, May 18, 2015

OLED! LED! Plasma(RIP)! On display technologies

OLED! LED! Plasma(RIP)! There are, today, quite a few technologies for video displays. What does it mean? What's best? Today we'll talk about video displays. This is an AV post, but if you're a "regular" user and curious about some of the buzzwords stick around.

Last week I stopped by at the Sapphire Marketing Roadshow as it traveled to my city of New York. Sapphire, as my AV friends know, is the rep firm for Crestron, Digital Projection, Vaddio, RPVisual, and Silicon Core. The latter was the brand I was most interested in seeing face-to-face. They might not be a household name outside the AV industry, but they have the most eye-catching product at this show: direct-view LED displays.

Plasma and LCD
Display technology has gotten confusing since the days when televisions were big and the only choice was a CRT screen. I'll assume you know that a video image is made up of tiny dots called pixels, and that a high-definition image measures 1920 pixels across by 1080 down. A "4K - UHD" display doubles each dimension, to 3840x2160 (the purists in the industry don't consider this "true" 4K as the horizontal pixel count is less than 4000 pixels. Aspect ratios are another topic). Today's question is what those pixels are, and how we light them up.

You can, without being too far wrong, think of a plasma display as an array of very tiny fluorescent lights (this is a metaphor. You'll remember that I like metaphors). Each element contains a phosphour which, when hit by an electron will admit a certain wavelength of light. Red, green, and blue phosphours will be combined to create the full range of color; when they're lit they're on, and when they're off they're off - more on this tautology later. Plasma is considered an emissive display technology because each elements emits light.

An LCD tv works differently. An LCD screen is a sort of glass sandwhich; the "bread" is a polarizing filter on the inside (facing the light source) and the liquid crystal on the outside (facing the viewer). In between are colored filters which give us our red, green, and blue pixels. An electrical charge applied to the liquid-crystal element would twist the crystal to align its polarization with the inner layer, letting light through. LCD displays aretransmissivein that each element allows light to be transmitted from a white source.

Which is better? This is a question I've heard a great many times before the slow death of plasma. For viewing experience, emissive technologies are better. Remember when I said that an element that was "off" is off? That means that "black" on a plasma display is truly black - no light. "Black" on an LCD will always have some leakage; it's a darkish-grey masquerading as black.

So why did plasma die? Plasma displays are very heavy. They use a great deal of energy. And they are susceptible to "burn in". Phosphours degrade as they emit light. This not only means that the entire image dims over time, but that displaying a static image may degrade one group of elements more than the rest, leaving a persistent "ghost image" on the display.

Edge-lit vs backlit (source as above)

Did I hear someone ask, "What about my LED TV"? LED TVs (not OLED or direct-view LED walls which we'll talk about later) are a special case of LCD display. Remember that "light source" I was talking about earlier? In older models that was a compact fluorescent bulb. IN newer ones, it's white LEDs, placed either at the perimeter of the viewing area (edge-lit) or behind it (back-lit). Back-lighting creates a brighter, more evenly lit image, while edge-lighting allows flat panels to be made almost absurdly thin.

OLED
Organic light-emitting diodes (or OLED) is an emissive technology, involving organic molecules which behave as LEDs. As with a plasma display, each color is directly emitted. Also as is the case with plasma, these elements will degrade with use. Your ten year old television? That would be functioning at less than half the brightness as when it was new if it used OLED. It's a great technology (in my opinion) for cell-phones. By the time your phone display has noticeably degraded you're very likely to have replaced it with a newer model for other reasons (processor speed, screen size, amount of memory, etc). You're even more likely to have dropped it and shattered the screen in the first place.

Direct-view LED

This brings us back to the Silicon Core display at the Sapphire roadshow; Silicon Core is one of several manufacturers of large-format emissive displays which use individual LEDs for each pixel. The challenge with those has always been "pixel pitch" or the distance between elements. Older LED displays, with a pixel-pitch over 2mm, are only well-suited for applications in which the viewer is very far away; closer, one sees "dots". We now can pack them in much closer; Silicon Core's "Lavender" display has a pixel pitch of 1.2mm. It's one of the prettiest displays I've seen, with two clear drawbacks:

Photos don't do this justice

First, there's cost. Not only are they expensive to purchase, they are expensive and difficult to mount. A large LED display of this type is made of many smaller (about a foot and a half square) blocks, each of which needs to be placed within a very tight tolerance to give a seamless-looking image. Because maintenance and connectivity is at the rear, this often involves a complicated "scissors" type mount with moving parts allowing for some kind of front-access.

Secondly, there's a manufacturer-specific issue with Silicon Core: their current "top of the line" model is called Lavender. Previous models, with larger pixel pitches, include Magnolia, Peony, Orchid, Sunflower, and Tulip. I'm afraid that before too long they'll run out of recognizable and pleasant flower-names, and we'll be stuck with "Stinkweed" or "Corpse Plant".